1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device which improved display grade through an improvement in contrast ratio.
2. Discussion of the Related Art
Recently, in connection with development an of information society, there have been demands for development of a novel image display device which improves drawbacks that a conventional cathode ray tube (CRT) has such as a heavy weight and a great bulk.
Under circumstances, various flat display devices such as LCDs (liquid crystal display devices), OLEDs (organic light emitting diodes), PDPs (plasma panel display devices), and SEDs (surface-conduction electron-emitter display devices) are getting attention.
Among these, liquid crystal display devices (LCDs), which are used in a variety of applications from a display screen of a mobile phone to a TV screen, are one of the most typical flat display devices.
LCD displays desired images by applying an electric field to a liquid crystal substance with dielectric and optical anisotropy between a pair of transparent substrates and adjusting the strength of the electric field to control the transmittance of incident light onto the substrates.
In general, an LCD includes a liquid crystal display panel for displaying images by receiving video and scan signals from outside and a backlight unit for irradiating light.
The liquid crystal display device includes a liquid crystal layer disposed between a pair of transparent substrates, and may further include a polarizing film equipped on the pair of substrates, respectively.
That is, the LCD is not a light-emitting display device which displays images by self-emitting light, rather it is a non-light-emitting display device which displays images by having light supplied from a backlight unit and controlling the transmittance of the supplied light.
In order to display high-definition images on the LCD, improvement in various image quality characteristics is in demand. Particularly, improvement in contrast ratio is important for displaying vivid images.
The contrast ratio is generally measured by dividing a white luminance by a black luminance, and particularly, reducing the black luminance is an important factor for improving the contrast ratio.
However, in an LCD, especially a normally black mode LCD, there is a problem in that a black luminance increases and a contrast ratio decreases by being unable to completely control incident light from a backlight unit when displaying a black screen.
When contrast ratio decreases, the contrast ratio of a white screen and a black screen decreases, thereby having difficulty in displaying vivid images.